The mammalian target of rapamycin (mTOR) is a 289 kDa serine/threonine kinase that is considered a member of the phosphoinositide-3-kinase-like kinase (PIKK) family, because it contains a carboxyl terminal kinase domain that has significant sequence homology to the catalytic domain of phosphoinositide 3-kinase (PI3K) lipid kinases. In addition to the catalytic domain at the C-terminus, mTOR kinase also contains a FKBP12-Rapamycin binding (FRB) domain, a putative repressor domain near the C-terminus and up to 20 tandemly-repeated HEAT motifs at the N-terminus as well as a FRAP-ATM-TRRAP (FAT) and FAT C-terminus domain. See, Huang and Houghton, Current Opinion in Pharmacology, 2003, 3, 371-377.) In the literature, mTOR kinase is also referred to as FRAP (FKBP12 and rapamycin associated protein), RAFT1 (rapamycin and FKBP12 target 1), RAPT1 (rapamycin target 1)).
mTOR kinase can be activated by growth factors through the PI3K-Akt pathway or by cellular stresses, such as deprivation of nutrients or hypoxia. The activation of mTOR kinase is thought to play a central role in regulating cell growth and cell survival via a wide range of cellular functions including translation, transcription, mRNA turnover, protein stability, actin cytoskeleton reorganization and autophagy. For a detailed review of mTOR cell signaling biology and potential therapeutic effects of modulating the mTOR signaling interactions, see Sabatini, D. M. and Guertin, D. A. (2005) An Expanding Role for mTOR in Cancer TRENDS in Molecular Medicine, 11, 353-361; Chiang, G. C. and Abraham, R. T. (2007) Targeting the mTOR signaling network in cancer TRENDS 13, 433-442; Jacinto and Hall (2005) Tor signaling in bugs, brain and brawn Nature Reviews Molecular and Cell Biology, 4, 117-126; and Sabatini, D. M. and Guertin, D. A. (2007) Defining the Role of mTOR in Cancer Cancer Cell, 12, 9-22.
Researchers studying mTOR kinase biology have discovered a pathological connection between the dysregulation of mTOR cell signaling and a number of diseases including immunological disorders, cancer, metabolic diseases, cardiovascular diseases and neurological disorders.
For example, there is evidence to show that PI3K-AKT signaling pathway, which lies upstream of mTOR kinase, is frequently overactivated in cancer cells, which subsequently results in the hyperactivation of downstream targets like mTOR kinase. More specifically, the components of the PI3K-AKT pathway that are mutated in different human tumors include, activation mutations of growth factor receptors and the amplification and overexpression of PI3K and AKT. In addition, there is evidence which shows that many tumor types, including glioblastoma, hepatocellular carcinoma, lung carcinoma, melanoma, endometrial carcinomas, and prostate cancer, contain loss-of-function mutations of negative regulators of the PI3K-AKT pathways, such as phosphatases and tensin homolog deleted on chromosome 10 (PTEN) and tuberous sclerosis complex (TSC1/TSC2), which also results in hyperactive signaling of mTOR kinase. The above suggests that inhibitors of mTOR kinase can be effective therapeutics for the treatment of diseases caused, at least in part, by the hyperactivity of the mTOR kinase signalling.
mTOR kinase exists as two physically and functionally distinct signaling complexes (i.e., mTORC1 and mTORC2). mTORC1, also known as the “mTOR-Raptor complex” or the “rapamycin-sensitive complex” because it binds to and is inhibited by the small molecule inhibitor rapamycin. mTORC1 is defined by the presence of the proteins mTOR, Raptor and mLST8. Rapamycin, itself, is a macrolide and was discovered as the first small molecule inhibitor of mTOR kinase. To be biologically active, rapamycin forms a ternary complex with mTOR and FKBP12, which is a cytosolic binding protein collectively called immunophilin. Rapamycin acts to induce the dimerization of mTOR and FKBP12. The formation of rapamycin-FKBP12 complex results in a gain-of-function, because the complex binds directly to mTOR and inhibits the function of mTOR.
A second, more recently discovered mTORC complex, mTORC2, is characterized by the presence of the proteins mTOR, Rictor, Protor-1, mLST8 and mSIN1. mTORC2 is also referred to as the “mTOR-Rictor complex” or the “rapamycin-insensitive” complex because it does not bind to rapamycin.
Both mTOR complexes play important roles in intracellular signaling pathways that affect a cell's growth, and proliferation, and survival. For example, the downstream target proteins of mTORC1 include Ribosomal S6 kinases (e.g., S6K1, S6K2) and eukaryotic initiation factor 4E binding protein (4E-BP1), which are key regulators of protein translation in cells. Also, mTORC2 is responsible for the phosphorylation of AKT (S473); and studies have shown that uncontrolled cell proliferation due to hyperactivation of AKT to be a hallmark of several cancer types.
Currently, several rapamycin analogues are in clinical development for cancer (e.g., Wyeth's CCI-779, Novartis' RAD001 and Ariad Pharmaceuticals' AP23573). Interestingly, the clinical data shows that the rapamycin analogs appear to be effective for certain cancer types, such as mantle-cell lymphoma, endometrial cancer, and renal cell carcinoma.
The discovery of a second mTOR protein complex (mTORC2) that is not inhibited by rapamycin or its analogs suggest that inhibition of mTOR by rapamycin is incomplete and that a direct mTOR kinase inhibitor which can inhibit both mTORC1 and mTORC2 at the catalytic ATP binding site can be more efficacious and have broader anti-tumor activity than rapamycin and its analogs.
Recently, small molecule mTOR inhibitors have disclosed, including in U.S. patent application Ser. Nos. 11/599,663 and 11/657,156 to OSI Pharmaceuticals Inc.; in International Applications WO/2008/023161 and WO/2006/090169 to Kudos Pharmaceuticals; and in International Applications WO/2008/032060, WO/2008/032086, WO/2008032033, WO/2008/032028, WO/2008/032036, WO/2008/032089, WO/2008/032072, WO/2008/031091 to AstraZeneca.
In view of the increased knowledge of the role of mTOR signaling in diseases (e.g., cancer), it is desirable to have small molecule inhibitors of mTOR (including mTORC1 and mTORC2) that can be used to treat diseases wherein aberrant mTOR activity is observed, such as, for example, in cancer. In addition, it can be desirable to have small molecule inhibitors of related enzymes (e.g., PI3K, AKT) that functions upstream or downstream of the mTOR signaling pathway.